Process for the preparation of hydrocarbon cracking stock for catalytic cracking



INVENTOR VIRGIL A. JOHNSON BY ,Zitm

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CRACKING STOCK FOR CATALYTIC CRACKING Sept. 23, 1952 Patented Sept. 23,1952 PROCESS FOR THE PREPARATION OF HY- DROCARBON ,CRACKING'STOCK FORCAT- ALYTIC CRACKING Virgil A. Johnson," Homewood, Ill., assignor toSinclair Refining Com corporation of Maine pany, New York, N. Y., a

Application February 9, 1949, Serial 75,356

My invention relates to improvements in the continuous preparation ofcatalytic cracking charge stock from distillate and residual stocksnormally available in refineries. More especially, it resides incontinuously providing feed stock to catalytic cracking at optimumreactor temperatures by separately preheating distillate and residualstocks, flashing the residual stock, and thereupon combining the flashedvapors with the preheated distillate stock and superheating the combinedstream to higher reactor temperatures than are possible withconventional handling.

My invention is designed to facilitate and improve the methods ofcontinuously preparing charge stock to catalytic cracking units underconditions obtaining in most petroleum refineries. Nearly all refineriesderive cracking stocks from a variety of sources; e. g., virgin orstraight run gas oils, recycle gas oils, reduced crude, and the like,and the heterogeneity of available stocks is characteristicallysubjected to the same pretreatment to clean up, preheat, and vaporizethem prior to charging to the various catalytic cracking processes. Ofcourse, it is essential to clean up catalytic cracking chargestocks-because regenerating capacity may be a prime limiting factor asin the fluid cracking process, or because operating efficiency andtiming cycles or good product distribution require clean chargingstocks.

Although refiners are severely pressed to obtain suitable feed stocks insuflicient quantity, capacity is limited by the desirability of avoidingpremature cracking before contacting the feed with the catalyst in thecracking reactor system. In this respect, the time the stock issubjected to cracking temperatures is the dominant factor. Whendistillate stocks are heated to higher temperatures, they vaporize andhave a high velocity through the heater so that the time factor andcorrelatively the severity of crack are reduced. The time factor,however, is higher with residual stocks due to the difliculty ofvaporizing them at operating pressures. Residual stocks, therefore, aresubjected to a more severe crack than lighter stocks under similartemperature conditions be,- cause of the increased time factor, withresulting coke formation causing interruption in heater operation.

lClaim. (oi. 196-52) Charge stock capacity is further limited by the incatalytic cracking charging stock preparation to run all stocks,regardless of nature or source,

in admixture through the feed preparation system. I have found that thislimits reactor'temperature's by restricting feed temperatures tothose'produced by superheating for a short time period vapors off thetarstrippers or flash towers used in charge stock preparation." Since flashtemperatures of combined streams containing residual stocks are limitedto about 800 to 850? F. to avoid excessive cracking, the temperaturepickup by superheating prior to introduction to the reactor is limitedto approximately another to F., or an increase to 950 to 10009 1 Thislimitation on reactor feed temperature is becom ingmore and morerestrictive as 'specialcatalysts permitting and making higherreactortemperatures economically desirable have 'attainedji'mportance. Ihave discovered, however, that increased capacity of the stripping orclean up section and higher reactor temperatures can be simultaneouslyobtained by a relatively simple' but strikingly beneficial procedure.Thus, I separately preheat distillate and residual stocks, separatelyflashing the residual stock to knock out tar and asphaltic matter, andthen combine the flashedvapors with the preheated distillate stockfor'delivery to cracking, usually after superheating the combined streamto the highest possible reactor temperature. In so doing, I takeadvantage of the fact that the usual blend of distillate charge stocks;e. g., virgin gas oil, recycle gas oil, and/or heavy naphtha may bepreheated to appreciably higher temperatures without cracking than theresidual and heavier stocks which must be cleaned up before charging tothe catalytic cracking unit. Thus, for example, I heat the distillatecharge stream to a temperature upwards of about 1050 F., while the flashtemperature of the residual stream is held at about 800 to 825 F. Thepreheated residual stream is ordinarily flashed, say at a pressure ofabout 25 p. s. i. g., and steam may be added to improve vaporization.When the flashed vapors are combined with the preheated distillatestream, full advantage is taken of the higher sensible heat in thedistillate stream, so that the combined vapor stream can be quicklysuperheated to the higher'reactor temperatures; e. g., upwards of about1050" to 1150 F., without undue cracking in view'of the reduction in thetime factor.

My improved method, will be illustrated in the accompanying drawingwhich provides by way of example one practical flow plan. The -residualstock; e. g., a reducedcrude is charged through line l0, picksup heat byheat exchanglfrom circulating hotstreams from otherunits in heat Recycle011i"; r

exchangers l I and i2 and is then passed through vaporizing heater l3 ofthe Deflorez type. As shown, a heavy side stream from a combination unitsecondary tower may be added to the reduced crude charge through line i4before heater 13. The preheated residual stream is then flashed in tarstripper l5 under conditions of mild pressure although obviouslypressure may be varied in this operation according to the stock handledand the desired overhead. Steam is usually admitted to the tower asthrough line IE to take advantage of the partial pressure effectinobtaining increased vaporization at lower temperatures. The bottomsare withdrawn through line H, while the flashed vapors pass overheadthrough line [8.

The distillate charge stream is usually made up of several light stocksas, for example, recycle gas oil charged through line 19 and straightrun or virgin gas oil charged through line 20. As shown the recycle gasoil is pumped hot from the cracking units while the straight run gas oilis preheated by heat exchange with circulating hot stream from otherunits in heat exchangers 2i and 22; The combined distillate stream ispassed through vaporizing heater 23 of the Deflorez type, and iscombined with the flash vapors in line 25. The combined vapor stream isthen superheated in superheater 25, and the superheated stream isdirectly conducted to the catalytic cracking system through line 26. V

The improved method will be furthcr illustrated, in the followingexamples comparing it with conventional operation. Ina system preparingabout 15,000 barrels per day feed for a Thermofor or moving bedtype-catalytic cracking unit, the feed consisted of reduced crude, aside stream from a combination unit secondary tower, recycle gas oil,and straight run gas oil and heavy naphthas from various refinerysources. Feed was'preheated ina'series of heat exchangers and thenpassed through two Deiiorez heaters in parallel. The-heater efiiuent wasflashed in the tar stripper at 820 F. and 25 p. s, i.'g., The gas oilwas taken ofi overhead, and'the vapor stream conducted to thesuperheater. Atthe superheater entrance process steam wa 'combined withthe tar stripper over- .head, and the combined stream was heated toabout 985 F. inthe superheater before passage to the reactor forcracking. The new rates-to ithe tar stripper are tabulatedloeleiw'v vGas on Bottoms, Overhead Bbls 1/ day- To Crack Feed,

Stream u /day Total; 10,990 5;

When'theabove unitwas modified to utilize my improved method,the'reduc'e'd, crude was preheated in a series of heat exchangers andcomstripper at 805 F. with about 5400 lbs./hour"of process steam.Thesteam was introducedinto zthe system at the tar stripperrather' thanat the superheater inlet in order to obtain lower flash temperatures inthe tar stripper. The straight run gas oil was preheated by heat exchange and combined with the recycle gas oil. The combined stream wassuperheated to about 1085 F. in the second vaporizing heater. This gasoil stream was then combined with the tar stripper stream at thesuperheater inlet and heated to 1095 F. in the superheater. Thesuperheater efliuent was passed to the reactor for cracking. The flowrates to the tar stripper and by passing the tar stripper are tabulatedbelow. It will be noted that the same amount of charge was prepared forcracking, but a significantly higher reactor feed temperature wasobtained which resulted in greater severity of cracking and betterconversion in the Thermofor unit.

How to tar stripper GasOil stream Feed, Bottoms, Overhead.

Bbls/d'ay Bbls/day To Cracking Reduced Crude r. 8, i00 5, 3, 270 #1G.-U. S. T S. S 2,020 2,020 #2 C. U. S. 'l. S. S 2,040 2,040

Total 12, 460 5,130 7, 330

Flow by passing- {or stripper V BblsJday, Stream Gas Oil to Cracking GasOil .2, 080 Thermal D chexanizer Bottoms r 1,020 Straight Run Naptha r090 Recycle Oil; r 3,740

Total .555

"Total gasoil'to cracking=7530 7330=14,86O hbls'./day. By modifying theabove described method by utilizing a higher flash temperature; i. e.,825 F., in the tar stripper, I'have found that both a higher reactor'feedtemperaturej i. e., 1110 F, and increased gas oil charge; i. '-'e.,580 bblst/da'y, for reactor feed are obtained. The following flow ratesand temperatures illustrate one example of thishigher temperaturemethod.

'Fl'o'u} to tar stripper oas Oii Overhead To Cracking Bbls/day StreamFlow by passing tar stripper Bblsllday, Gas Oil to Cracking Stream Gasoil; 2,080

Straight Run Naphtha Recycle on 3,140

Total 7,530

Flash: 825 F :1110 F.

Hence my improved method of preparing catalytic cracking charge stocksprovides an operation which can be used for any type'catalytic crackingsystem when the cracking feed stock is obtained from various refinerysources and a tar stripper is used to prepare feed continuously forcracking purposes. My improved system obviates overloading of the tarstripper and yields a cleaner stock to cracking, while simultaneouslyproducing higher reactor feed. temperatures, e. g. 1095 F. compared toabout 985 F. maximum in conventional operation, allowing added crackingseverity which can be profitably utilized with present catalysts.Moreover, in certain operations, the amount of gas oil feed availablefor cracking from present cracking stocks can be significantly increasedwhile developing the higher reactor feed temperatures.

I claim:

In the continuous preparation of catalytic charge stock from residualstock having components which tend to crack and form coke upon heatingto a high temperature and from distillate stock, the improvement whichconsists of sepa- I rately preheating the residual stock to a moderatetemperature of about 800 F., flashing said preheated residual stock toknock out tar and asphaltic matter and to produce residual stock vaporsat a temperature of about 800 F., separatelyvaporizing said distillatestock by pre-- heating to a higher temperature upwards of about 1050 F.,combining the relatively low temperature residual stock vapors with therelatively high temperature distillate stock vapors to elevate thetemperature of the residual stock vapors and to lower the temperature ofthe distillate stock vapors, rapidly passing the combined stream ofdistillate and residual stock vapors through a superheating zone toraise quickly the temperature of the combined stream to upwards of about1080 F. thereby avoiding undue cracking and coking of the residual stockvapors, and

conducting the superheated vapor stream to the catalytic crackingreaction system.

VIRGIL A. JOHNSON.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,166,176 Peterkin July 18,19392,360,349 Kassel Oct. 1'7, 1944

